Linux System Administration

Postfix, Sendmail, and Other MTAs

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Key Mail Service Terms

Mail transfer agents do the heavy lifting of Internet communication, moving mail

from site to site on the Internet. To send mail, an email sender attaches his system to

an MTA, which then uses SMTP to transfer the mail to the MTA responsible for

delivering mail to the recipient.

The recipient has several ways to retrieve mail from the MTA, none of them using

SMTP: she can log in as a user on the system that runs the MTA, attach to the MTA

through a direct connection (such as a dial-up line to an ISP), or tunnel though the

Internet to a remote MTA. (We’re ignoring methods that are further removed, such

as retrieving email through a web interface such as Gmail or using a cell phone.)

Regardless of which of these methods the recipient uses, she retrieves her mail

through a mail delivery agent (MDA) such as Courier IMAP. The MDA talks to the

MTA to get the mail and provides an inbox so she can collect her mail. Mail can then

be displayed on the user’s system through a mail user agent (MUA), such as Out-

look, Evolution, or Thunderbird.

Users typically retrieve email using either POP3 or IMAP4 over TCP/IP. Virtually all

modern MUAs support both POP3 and IMAP4. MUAs send mail by attaching to

MTAs and transferring the mail over SMTP.

Most people keep address books listing their contacts so their MUAs can look up

people’s email addresses. In